DSP56857 datasheet(319/662 Pages) FREESCALE

319 / 662 page

Functional Description

Enhanced Synchronous Serial Interface (ESSI), Rev. 4

Freescale Semiconductor

12-19

shown in Figure 12-6. If it is desired to run off a single set of interrupts, the TX interrupts should

be used. If RX interrupts are used, there may be timing problems with the transmit data because

this interrupt occurs a half-bit time before the transmit data is used by the hardware.

Figure 12-6. Synchronous Mode Interrupt Timing

12.5.4 Network Mode with Mask Registers Implemented

In order to reduce interrupt overhead, a number of enhancements were made to the ESSI module.

The enhancements incorporate the mask registers (TSM and RSM).

Together, Figure 12-8 and Figure 12-33 illustrate sample timing of Network mode transfers

using the mask registers. These figures duplicate the frame structure of Figure 12-5 to illustrate

the reduced amount of interrupt processing required. The numbered circles and arrows in the

figures identify discussion notes contained in Table 12-10 and Table 12-11.

12.5.4.1 Operation Using TSM Register

When the TSM register is included in the design interrupt overhead can be reduced. If all bits of

the TSM register are set, the ESSI transmitter will continue to operate as previously described.

The TSM register is used to disable the STD pin on specific time-slots. This is accomplished by

writing the TSM with 0 in the time-slot bit location. Disabling a time- slot in this manner causes

the time-slot to be ignored by the ESSI. This means no data is transferred to the Transmit Shift

using TSM registers for an 8-bit word, continuous clock with disabled FIFO five words per frame

sync in the Network mode is illustrated in Figure 12-7. Explanatory notes for the transmit

portion of the figure are provided in Table 12-10.

SCK

SC2

STX Register

TDE Status Bit / Interrupt

RDR Status Bit / Interrupt

Valid

Indefinite transition depends on SW interrupt processing

Invalid

TX Interrupt

RX Interrupt

TX Interrupt

RX Interrupt

DSP56857 Datasheet(PDF) 319 Page - Freescale Semiconductor, Inc